The detection and identification of objects within a turbid medium has long challenged investigators from diverse technical disciplines. For example, at visible frequencies objects or vessels located in fog are rendered largely undetectable, thus inhibiting safe navigation. In material science methods have long been sought to remotely image faults or other inhomogenieties within optically transluscent or opaque materials.
Objects buried in a random medium are rendered optically invisible because of the effects of scattering. Scattering causes light to experience multiple paths thereby obscuring phase information.
In a general sense the detection or imaging of subsurface objects in a random medium requires the solution of an inverse problem. Specifically this requires an understanding of how the medium affects the propagation of light. Mathematically this can be described by a modulation transfer function.
While prior art techniques have allowed observers to detect the presence of an object within a turbid medium (see below), none have allowed the observer to detect its depth or structure. In clinical medicine, for example, it would be useful to detect the presence, size, location, and depth of a suspected tumor.
In particular, it would be especially desirable to perform such measurements in a manner which could also monitor the close association between oxidative metabolism and organ function. As described below, this relationship can be determined by measuring the oxygenation or redox-state of heme proteins in situ (e.g. hemoglobin, myoglobin, cytochrome oxidase) using optical transmission or reflectance techniques.
An optical technique which could differentiate depth-of-field may be used to generate a 3-dimensional image of body tissues which identifies the oxygenation state of those tissues. Such measurements would thus permit the 3-D imaging of the functional activity of the tissue. Indeed, the development of such a technique would be useful for other applications including detecting and imaging objects located within the atmosphere or, in oceanographic studies for the imaging of submerged vessels or other subsurface objects.
Because of the strong scattering properties of tissues, it is often not practical to study their optical properties by a transmission measurement. On the other hand, by resorting to a reflectance or backscatter measurement the spectroscopic properties of tissue can be studied.
Much interest in this technique has been generated in recent years because of its ability to monitor, in a continuous and non-invasive fashion, in situ, the oxygenation state of heme proteins. Such measurements are highly desirable because of the well established relationship between oxygen availability, oxidative metabolism and organ function.
Using reflectance techniques, the effects that hypoxic hypoxemia, hypercapnia, carbon monoxide, and cyanide induced hypoxemia have on cerebral and cardiac energetics can be monitored and compared to simultaneous measurements of organ function.
Such studies demonstrate the extreme sensitivity of organ energetics and function to minimal changes in tissue oxygen levels as measured by the oxygenation state of heme proteins. The latter findings are consistent with results obtained using phosphorus NMR. Recent clinical investigations employing NIR reflectance measurements have demonstrated that rapid changes in brain oxygenation, blood volume and energy state occurred in response to hyperoxia, moderate hypoxia and hypercapnia. These investigations emphasize the clinical usefulness of this technique.
Although it has been recognized that optical studies of tissue (particularly in the reflectance mode) can potentially yield highly significant and useful information regarding organ function, such measurements are of little clinical value without the ability to characterize some degree of depth-of-field.
In fact, depth-of-field differentiation is critical. Without a depth-of-field differentiation these techniques cannot even distinguish between a superficial bruise on the scalp and a more serious internal injury.
The inability to differentiate depth-of-field in tissue by past optical measurement techniques has compelled the prior art to restrict measurements to the identification of the overall oxygenation state of the tissue (i.e., solely one dimensional information). For example, U.S. Pat. No. 4,281,645 (Jobsis) discloses a spectrophotometric transillumination method for non-invasive monitoring of the metabolism of a body organ which performs this type of one-dimensional measurement. Jobsis also discussed this prior art technique in Reflectance Spectrophotometry and Non-Invasive, Infrared Monitoring of Cerebral and Myocardial Oxygen Sufficiency and Circulatory Parameters (1977). Such spectrophotometric techniques are not able to use the dispersion pattern generated by objects buried in the target medium to image the objects and establish their location within the surrounding target medium.
Prior attempts have been made to locate and image objects located within a turbid medium by utilizing transillumination techniques, for example, diaphanography, where light is directed towards a target object. In transillumination the target object is detected by shadows in the light pattern emerging at the opposite side of the medium.
Typical of such prior attempts include inventions disclosed by United Kingdom Patent Application Nos. 2,068,537A, 2,111,794, 2,154,731, and 2,092,856 and U.S. Pat. No. 4,312,357. Such transillumination techniques only permit an observer to determine the two-dimensional outline of the target object. Thus its depth within the surrounding tissue and its three-dimensional structure remain unknown. Furthermore, these transillumination techniques require that a specimen be relatively thin and that both sides of the specimen be accessible to the measurement device.
U.S. Pat. No. 4,555,179 (Langerholc et. al.) discloses a method and apparatus for the detection of objects in a scattering medium. The Langerholc technique employs a collimated light source which scans the medium. The reflected radiation is then analyzed to detect the presence of the target object. If the absorption characteristics of the target object were previously known, the observer could determine the object's depth within the medium. For clinical studies, such information can not be practically obtained.
A further serious drawback of the Langerholc method is that, in practice, the method described can only locate objects relatively close to the surface of the medium being scanned. In addition, unlike the current invention, the measurement described involves integrating the backscattered signal over an area having a diameter equal to 2.5 times the thickness of the medium (i.e. the so-called "scattering zone"). This restriction renders such an approach useless for measurement of targets having limited geometries, i.e. the human body. Furthermore, by integrating over this area, specific information regarding variations in the position and angle dependent emerging flux is lost.
On the other hand, the present invention recognizes the significance of performing a position and angle scan of the scattered light as essential in identifying regional variations in the absorptive and scattering properties of a turbid medium. Indeed, discussions of techniques such as Langerholc's admit that they are only able to detect objects along one dimension. These include wires or blood vessels. This is of limited value when the observer wishes to know the location, size, and shape of three-dimensional objects within a turbid medium.
Bonner et al., in Model For Photon Micration in Turbid Biological Medium (1987), recognized that analysis of positional information can be used to infer sub-surface properties of a random medium. However, Bonner does not disclose how such information may be used to generate an image of the target medium.
While prior art techniques have allowed observers to detect the presence of an object within a turbid medium, none have allowed the observer to image its depth or structure. In many medical and other applications, such information is vital.
The present invention describes an imaging technique which may be used to study the brain and breast. The presence of tumors, cysts, hypoxic or infracted regions will be readily detectable. For obstetrical procedures, it will be possible to image the oxygenation state of an unborn fetus's brain, in utero, just prior to delivery. The progression of atherosclerosis on the delivery and utilization of oxygen by tissues (in particular, limbs) could be directly assessed. Because the technique of the invention is noninvasive and nondestructive and yields vital physiological information, it will also be useful in monitoring the response of body tissues to various therapies. In particular, the technique of the present invention may be used to monitor and evaluate the physiological status of burned patients, immuno-compromised patients or other patients restricted to isolation rooms.
The inventive technique represents an accurate and reliable means to assess the functioning of transplanted organs as well as being a sensitive means to detect, in situ, impending organ rejection. The disclosed technique wil be helpful in monitoring the physiological status of excised donor organs for impending recipient implantation. Its use during anesthesia will permit, for the first time, direct in situ monitoring of the delivery to and utilization of oxygen by the brain. Such measurements in a hospital emergency room setting with unconscious patients would differentiate accurately and rapidly between patients suffering from carbon monoxide-, cyanide-, or other drug poisonings which interfere with oxygen utilization from patients having suffered a stroke or subtle cerebral trauma.
Studies in marine environments using visible and/or near infrared (NIR) sources will permit the remote monitoring of the oxygenation status of plants and animals, in situ. Such measurements will indicate the physiological impact of environmental pollutants. Other marine studies may involve the imaging of turbulence at increasing ocean depths. The proposed method may also be used in search and rescue operations for the imaging of objects or victims buried in snow, ice or muddy waters.
In a general sense, the above applications pertain to the imaging of objects which exist in the medium at the time of measurement. The present technique may also permit the determination of events which have previously occurred and thereby affected changes in the environment. This is particularly useful for the measurement of affects on marine life.
Commercial applications of the proposed technique include the remote inspection of food products for spoilage or contamination by insecticides. As a remote imaging method, the proposed technique might aid aircraft or naval navigation in foggy atmospheres. Other commercial applications include the monitoring of various processes involved in the production of bulk industrial commodities at critical steps involving significant hazards to operators. The remote imaging of optically opaque objects which are not readily explored by a transmission measurement may also be imaged. Examples include the interrogation of low fault tolerant components of existing structures for the detection of fault lines or other fractures by the use of x-ray or particle beam sources. These may include nuclear reactor shielding, hulls of commercial or military aircraft, etc.
Consequently, it is an object of this invention to provide a method for allowing an observer to accurately detect, three-dimensionally image and spectroscopically characterize target objects located within a turbid medium.
Another object of the present invention is to employ radiation directed towards target objects located in a random medium and to detect radiation scattered from the medium to enable the observer to determine the object's depth, structure, absorptive and scattering properties within the turbid medium.
A further object of this invention is to image reference objects in a non-invasive and non-destructive manner.
Yet another object of the present invention is to provide a method whereby a physician may use the invention to aid in medical diagnosis.
For example, a goal of this invention would be to measure the oxygenation state of body tissues and to display this information as a three-dimensional image to yield vital physiological information while still being a sensitive indicator of subtle physiological stress caused by disease or trauma.
Additional objects and advantages of the invention will be set forth, in part, in the following description, will be obvious, in part, from this description, or may be learned from the practice of this invention. The objects and advantages of the invention are realized and obtained by the processes and methods particularly pointed out in the following description and claims.